The invention relates generally to methods of fabricating semiconductor materials, and more particularly to a method of forming a semiconductor substrate for use in the manufacture of integrated circuit devices, the substrate including a buffed insulating layer for electrically isolating surface regions from underlying supporting regions of the substrate.
An area of ongoing research in the manufacture of large-scale integrated circuits is the use of silicon wafers processed to include a buried insulating layer beneath the wafer surface. One technique for forming silicon substrates with a buried insulating layer is referred to by the acronym SIMOX (for "Separation by IMplanted OXygen"). In SIMOX processing a monocrystalline silicon wafer is implanted with a beam of oxygen ions accelerated at high energies and directed through the surface of the wafer. The oxygen ions come to rest at a selected depth within the wafer substrate. The result is a buffed region of implanted oxygen extending between upper and lower layers of monocrystalline silicon. The silicon wafer is then annealed, causing a redistribution of the oxygen ions to form a substantially uniform buffed layer of silicon dioxide (SiO.sub.2) in the wafer. Annealing tends to sharpen the demarcation between the buffed layer of silicon dioxide and the adjacent layers of monocrystalline silicon, and annealing also repairs the damage to the crystalline structure of the upper or superficial silicon layer, which is damaged by the ion implantation.
The buried layer of silicon dioxide which results from SIMOX processing improves the performance of integrated circuit formed on the wafer. That is because the oxide electrically isolates the surface layer of silicon, where semiconductor devices are fabricated, from the bulk portion of the wafer beneath the silicon dioxide layer. The silicon dioxide tends to minimize capacitance between the active devices on the surface and the supporting substrate. It also helps prevent the development of electrical paths through the wafer which can degrade or destroy surface devices.
One problem associated with SIMOX processing is that the ion bombardment and subsequent sealing can leave broken Si--O and Si--Si bonds buried within the substrate, generally adjacent the interface regions between the silicon dioxide layer and the upper and lower silicon layers. Such broken bonds are a source of free electrons and trapped electric charge in the substrate which can cause undesired current paths to develop between active devices and the supporting wafer. Trapped electrical charge lowers the breakdown voltage of the buffed oxide and reduces its hot electron immunity. As such, broken molecular bonds in the interfacial regions around the buffed oxide layer produce an increase in defects in integrated circuits formed on the wafer, can cause poor device performance, and lowers the overall quality of the fabricated wafer.
Nitrogen passivation is a technique which is used to counter the harmful effects of broken molecular bonds and trapped electrical charge in semiconductor wafers. "Passivation" involves the introduction of free nitrogen ions into the vicinity of defective molecules, which allows the nitrogen to join and form stable bonds with the broken molecules, reducing the number of free electrons. Surface regions of a silicon substrate, such as the gate oxide region, can be readily passivated by nitrogen diffusion or shallow implantation. If devices are formed on a substrate that includes a surface layer of deposited polysilicon, the introduction of nitrogen by such shallow-penetration techniques has little or no harmful effect on device performance. Polysilicon in active devices is highly tolerant of nitrogen. However, active devices formed in monocrystalline silicon are seriously degraded by the presence of nitrogen. Therefore, it would be advantageous to employ a methodology for passivating the interface regions adjacent a buffed oxide insulating layer which does not introduce undesirable nitrogen into the surface silicon layer.
It would also be advantageous to be able to increase the electrical isolation between the upper and lower semiconductor regions in a silicon wafer manufactured using the SIMOX methodology.
It would also be advantageous to improve the buried oxide breakdown voltage in a SIMOX substrate without degrading the gate silicon incorporated into active devices fabricated on the substrate.
Accordingly, a method of increasing the electrical isolation in a semiconductor substrate is provided. The method is used in a substrate of the type which, when processing is completed, includes a buffed insulating layer formed between upper and lower semiconductor regions, the buffed insulating layer being centered at a depth D beneath the top surface of the substrate. Increased electrical isolation between the upper and lower semiconductor regions, in accordance with the invention, is provided by the following steps: (a) implanting nitrogen ions into the substrate to the depth of the buried insulating layer; and (b) heating the substrate to cause migration of nitrogen ions to the interface regions extending between the buried insulating layer and the upper and lower regions of the semiconductor substrate. The result of the method is passivation of the interface regions by the nitrogen, thereby increasing the electrical isolation between the upper and lower semiconductor regions.
In its preferred form, the method is carried out in conjunction with SIMOX processing of a silicon wafer substrate. SIMOX requires the implantation of oxygen ions in the substrate to a selected depth below the top surface of the substrate. After oxygen implantation, the substrate is annealed to form a buried insulating region of oxide between upper and lower semiconductor regions within the substrate. During SIMOX annealing, the implanted oxygen ions are redistributed and bond to the silicon of the substrate, forming the buried silicon dioxide layer characteristic of SIMOX. The method of the present invention can be carried out as part of the SIMOX process. The aforementioned, step (a) of implanting nitrogen ions into the substrate can be performed before or after the implantation of oxygen ions in the SIMOX process, or substantially simultaneously with the oxygen implantation. The aforementioned heating step (b), in the method of the present invention, can be performed during the annealing step of the SIMOX process. In conjunction with the present invention, SIMOX annealing causes migration of the implanted nitrogen ions to the interface regions extending between the buried silicon dioxide layer and the upper and lower semiconductor regions of the substrate.
An alternative embodiment of the method of the present invention allows for completion of the SIMOX process prior to nitrogen ion implantation. In this alternative, a monocrystalline silicon substrate is implanted with oxygen ions and annealed to form a buried silicon dioxide region within the substrate. Subsequently, the aforementioned step (a) of implanting nitrogen ions, and step (b) of heating the substrate, are performed. In this alternative, the heating step (b) is performed separately from the SIMOX annealing step since the SIMOX annealing step was completed before nitrogen implantation. Instead, the substrate is heated in a separate step (b) to cause migration of the nitrogen ions. Or the separate heating step (b) can be carried out during the subsequent processing of the substrate to form active devices on its surface.
One use of the invention is to form active semiconductor devices on a monocrystalline silicon substrate processed in accordance with SIMOX techniques. The method allows the formation of one or more active devices, such as MOS FETs, on a substrate with a nitrogen-passivated buried oxide layer, wherein the channel region of the devices are substantially free of implanted nitrogen. The channel regions are therefore not degraded by the presence of nitrogen, which otherwise adversely affects the performance of devices formed in monocrystalline silicon.
The preferred implantation energies and doses used in the nitridation of SIMOX buried oxide process of the present invention are as follows: oxygen ions are preferably implanted to a selected depth D using an implant dose generally in the range of 1.0.times.10.sup.17 to 3.0.times.10.sup.18 ions/cm.sup.2 with an ion implantation energy generally in the range of 30 KeV to 120 KeV. Nitrogen ions are implanted to generally the same depth as the depth D of the oxygen ions using an implant dose generally in the range of 1.0.times.10.sup.11 to 1.0.times.10.sup.13 ions/cm.sup.2 with an ion implantation energy which is generally in the range of between zero percent (0%) to twenty five percent (25%) less than the ion implantation energy used to implant the oxygen ions. The annealing temperature and duration used with the present invention are conventional for SIMOX processing. If the substrate is heated subsequent to SIMOX annealing, in order to cause migration of the nitrogen ions to the interfacial regions between the oxide layer and the semiconductor layers, the heating step generally requires heating the substrate to between 700.degree. C. and 1350.degree. C. for between 10 minutes and 12 hours. Preferably, the heating step, following SIMOX annealing, is performed by heating the substrate to a temperature generally in the range of 700.degree. C. to 1000.degree. C. for between 10 minutes and 1 hour.